CaO Surface Research Articles

The phenol steam reforming reaction towards H2 production on natural calcite

The steam reforming of phenol towards H2 production was studied in the 650–800°C range over a natural pre-calcined (air, 850°C) calcite material. The effects of reaction temperature, water, hydrogen, and carbon dioxide feed concentrations, and gas hourly space velocity (GHSV, h−1) were investigated. The increase of reaction temperature in the 650–800°C range and water feed concentration in the 40–50vol% range were found to be beneficial for catalyst activity and H2-yield. A similar result was also obtained in the case of decreasing the GHSV from 85,000 to 30,000h−1. The effect of concentration of carbon dioxide and hydrogen in the phenol/water feed stream was found to significantly decrease the rate of phenol steam reforming reaction. The latter was probed to be related to the reduction in the rate of water dissociation as evidenced by the significant decrease in the concentration of adsorbed bicarbonate and OH species on the surface of CaO according to in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS)-CO2 adsorption experiments in the presence of water and hydrogen in the feed stream. Details of the CO2 adsorption on the CaO surface at different reaction temperatures and gas atmospheres using in situ DRIFTS and transient isothermal adsorption experiments with mass spectrometry were obtained. Bridged, bicarbonate and unidentate carbonate species were formed under CO2/H2O/He gas mixtures at 600°C with the latter being the most populated. A substantial decrease in the surface concentration of bicarbonate and OH species was observed when the CaO surface was exposed to CO2/H2O/H2/He gas mixtures at 600°C, result that probes for the inhibiting effect of H2 on the phenol steam reforming activity. Phenol steam reforming reaction followed by isothermal oxygen titration allowed the measurement of accumulated “carbonaceous” species formed during phenol steam reforming as a function of reaction temperature and short time on stream. An increase in the amount of “carbonaceous” species with reaction time (650–800°C range) was evidenced, in particular at 800°C (4.7 vs. 6.7mgC/g solid after 5 and 20min on stream, respectively).

Adsorption Energies of Mercury-Containing Species on CaO and Temperature Effects on Equilibrium Constants Predicted by Density Functional Theory Calculations

The adsorption of Hg, HgCl, and HgCl2 on the CaO surface was investigated theoretically so the fundamental interactions between Hg species and this potential sorbent can be explored. Surface models of a 4 x 4 x 2 cluster, a 5 x 5 x 2 cluster, and a periodic structure using density functional theory calculations with LDA/PWC and GGA/BLYP functionals, as employed in the present work, offer a useful description for the thermodynamic properties of adsorption on metal oxides. The effect of temperature on the equilibrium constant for the adsorption of mercury-containing species on the CaO (0 0 1) surface was investigated with GGA/BLYP calculations in the temperature range of 250-600 K. Results show that, at low coverage of elemental mercury, adsorption on the surface is physisorption while the two forms of oxidized mercury adsorption undergo stronger adsorption. The adsorption energies decrease with increasing coverage for elemental mercury on the surfaces. The chlorine atom enhances the adsorption capacity and adsorbs mercury to the CaO surface more strongly. The adsorption energy is changed as the oxidation state varies, and the equilibrium constant decreases as the temperature increases, in good agreement with data for exothermic adsorption systems.

Transition metal atoms on oxide supports density functional calculations

AbstractThe adsorption properties of Cu, Ag, Ni, and Pd atoms on O2−, F, and F+ sites of MgO, CaO, SrO, and BaO (001) surfaces have been studied by means of density functional calculations. The examined clusters were embedded in the simulated Coulomb fields that closely approximate the Madelung fields of the host surfaces. The adsorption properties have been analyzed with reference to the basicity and energy gap of the oxide support in addition to orbital interactions. While the free Ni d9s1 triplet ground state is preserved on adsorption on the O2− sites of MgO, CaO, and SrO surfaces, it is no longer preserved on the O2− site of BaO. For all adsorbates considered, adsorption is found to be stronger on F+ sites compared with regular O2− sites. While on the O2− site, Pd and Ni form the most stable complexes, on the F site, Pd and Cu form the most stable counterparts. On the F+ site, the single valence electron of Cu and Ag atoms couples with the unpaired electron of the vacancy forming a covalent bond. As a result, the adsorption energies of these atoms on the F+ site are stronger than those on the F and O2− sites. The adsorption properties correlate linearly with the basicity and energy gap of the oxide support in addition to orbital interactions. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009

Surface water reactivity of polycrystalline MgO and CaO films investigated using x-ray photoelectron spectroscopy

The authors report a study comparing the surface reactivities of the alkaline earth oxides MgO and CaO with respect to water vapor under ultrahigh and high vacuum conditions. Using x-ray photoelectron spectroscopy and a series of in vacuo exposures spanning ∼10−10Torr vacuum, to 10−6Torr flowing oxygen, the extent of surface hydroxylation was investigated by monitoring the O 1s photoelectron line. After the most aggressive exposures, the MgO surface reacted to form a maximum hydroxyl coverage of ∼1.3 ML (monolayer), while the CaO surface reached a maximum coverage of ∼3.0 ML. Both surface hydroxides could be removed by in vacuo thermal anneals; exposure to 250 and 500°C were required for MgO and CaO, respectively. These results are framed in terms of the suitability of these oxides in complex oxide-compound semiconductor heteroepitaxial multifunctional devices and with respect to understanding the development of surface morphology in epitaxial 111-oriented CaO and MgO thin films.

The adsorption of CO2 and CO on Ca and CaO films studied with MIES, UPS and XPS

Metastable Induced Electron Spectroscopy (MIES), Ultraviolet Photoelectron Spectroscopy (UPS), and X-ray Photoelectron Spectroscopy (XPS) are employed to study the adsorption of CO2 and CO on Ca and CaO films. Ca films are prepared by evaporation of Ca onto clean Si(100) substrates. CaO films are produced by Ca evaporation in an oxygen atmosphere at a substrate temperature of 670K. CO2 interaction with the Ca films is initiated by dissociation of the impinging molecules leading to the formation of Ca–O bonds. These Ca–O bonds are subsequently consumed in the formation of a closed CaCO3 layer on top of the surface. CO interaction with the Ca surfaces also leads to the dissociation of the molecule and the formation of Ca–O bonds. We find evidence for the subsequent formation of CO32- complexes on top of the surface. On CaO surfaces, both CO2 and CO lead to the formation of a closed CaCO3 top layer, though displaying very different reaction rates.

Mercury Species and SO2 Adsorption on CaO(100)

First principles-based quantum mechanical tools based upon Density Functional Theory were used to investigate the binding mechanism of Hg species and SO2 on CaO(100) surfaces for parallel and perpendicular orientations. One-fold, 2-fold and 3-fold high symmetry adsorption sites have been examined for the species, Hg0, SO2, HgCl, HgCl2 and HgO. It has been discovered that HgCl, HgCl2, and SO2 are strongly adsorbed on the CaO(100) surface at 0.125 ML coverage with chemisorption as the likely adsorption mechanism. Binding energies of Hg0 are minimal indicating a physisorption mechanism.

Electronic energy transfer on CaO surfaces

We excite low-coordinated surface sites of nanostructured CaO samples using tunable UV laser pulses and observe hyperthermal O-atom emission indicative of an electronic excited-state desorption mechanism. The O-atom yield increases dramatically with photon energy, between 3.75 and 5.4 eV, below the bulk absorption threshold. The peak of the kinetic energy distribution does not increase with photon energy in the range from 3.9 to 5.15 eV. These results are analyzed in the context of a laser desorption model developed previously for nanostructured MgO samples. The data are consistent with desorption induced by exciton localization at corner-hole trapped surface sites following electronic energy transfer from higher coordinated surface sites.

Theoretical study of different speciation of mercury adsorption on CaO (0 0 1) surface

Adsorption of mercury and mercury chloride on the surface of CaO surface has been investigated by means of density functional theory (DFT) cluster model calculations. Different embedding clusters (shell models, point charges cluster and bare cluster) have been compared. The calculation functions and basis used for mercury were optimized to provide the reliable fundamental understanding of gas–solid adsorption system. A small binding energy of −21.47 kJ/mol was found when the elemental mercury molecule is coordinated to the O 2− anion, which inferred mercury was weakly physisorbed at the surface of the CaO. HgCl 2 adsorption is more complex. When the mercury chloride molecular axis was parallel to the surface, the Hg atom oriented to the O 2− anion and had a large binding energy, which meant that mercury chloride adsorbed on surface of CaO is mainly chemisorption. The present calculations indicated that CaO injection could substantially reducing gaseous mercury chloride, but have no apparent effect on the mercury, which was compatible with the available experimental results. Besides obtaining structures and energies, the temperature effect on adsorption were also studied. The results suggested that the high temperature has adverse effect on the adsorption when the temperature is above 280 °C because of the nature of adsorption.

Cl 2 adsorption on supported alkali metals and on the MgO and CaO (001) supports: A DFT study

The adsorption properties of Cl 2 molecules adsorbed on Li and Na atoms supported on various sites of MgO and CaO (100) surfaces have been examined at the DFT/B3LYP level of theory using the embedded cluster model. By comparison, the adsorption on the clean surfaces of the support has been investigated. Our calculations predict weakening of the Cl 2 bond due to the presence of the alkali metals, reflected by an elongation of the Cl 2 bond by 0.64 Å. Although the regular and defective sites are stronger binding sites in CaO (001) than in MgO (001), the interaction energy decreases as the atomic number of alkali atoms (Li and Na) increases. Metal atoms bind more strongly on the defect sites. The MgO and CaO (001) surfaces not only serve as the supports of Li and Na atoms but also participate in the interaction with Cl 2 molecule. Charge transfer takes place from the oxide surface to the adsorbed metal.

Mesoporous CaO–ZrO 2 nano-oxides: A novel solid base with high activity and stability

A new solid base, mesoporous CaO–ZrO 2 nano-oxides, was prepared via appropriate sol–gel process. It was found that such solid base showed high basicity and stability. Temperature programmed desorption of CO 2 confirmed that both weak and strong basic sites coexisted on the surface of CaO–ZrO 2 and the basicity could be modified by adjusting Ca/Zr molar ratio of the nano-oxides. The mesoporous CaO–ZrO 2 nano-oxides showed high thermal stability, since X-ray diffraction (XRD) and transmission electron microscopies (TEM) proved that the mesoporous network of this material preserved well even up to 700 °C. At the same time, XRD and Raman spectra indicated that Ca 2+ incorporated into t-ZrO 2 lattice and the basic sites were firmly anchored to the substrate. As a result, this novel solid base showed remarkable activity and durability in the synthesis of dimethyl carbonate from methanol and propylene carbonate.

Predicted Changes to Alkali Concentration Adjacent to Glass–Crystal Interfaces

Atomic scale molecular dynamics simulation is used to investigate how alkali concentrations in glass adjacent to interfaces with crystal surfaces, might differ from those within the interior of a bulk glass. A sodium, lithium alkali-borosilicate glass and its interfaces with the (1 0 0) and (1 1 0) surfaces of MgO, CaO, and SrO are considered. These six interfaces were formed by melt-quenching the glass in the presence of the crystal surface, providing glass alkali ions with the opportunity to either migrate towards or away from the crystal surface. The resulting alkali contents adjacent to the interface are found to differ depending on crystal composition and surface orientation.

Adsorption of dimethyl methylphosphonate and trimethyl phosphate on calcium oxide: an ab initio study

Ab initio study of the adsorption of dimethyl methylphosphonate (DMMP) and trimethyl phosphate (TMP) on three types of models simulating the calcium oxide surface (non-hydroxylated Ca4O4, completely hydroxylated Ca4O4(OH)2H2, and partially hydroxylated Ca4O4(OH)H) was performed. The target molecule and the surface hydroxyl groups were optimized while the CaO fragment was kept frozen. The intermolecular interactions were investigated applying Bader’s Atoms in Molecules theory. The maps of electrostatic potential of the studied adsorption systems were also produced. The interaction energies of studied adsorption systems corrected by the basis set superposition error were obtained. The most energetically favorable adsorption of DMMP and TMP was found at the configuration where the oxygen atoms of the P=O and methoxy groups point toward the Ca cation of the surface. The P atom points toward the O atom of the surface and forms a P–O chemical bond. This configuration was revealed for the non-hydroxylated and partially hydroxylated CaO-DMMP and CaO-TMP systems. The presence and number of surface hydroxyl groups on the CaO models play a key role in the adsorption of the studied compounds. DMMP and TMP were found to be much less stable on the completely hydroxylated CaO surface where they are adsorbed only via weak electrostatic interactions and H-bonding to the surface oxygen atoms and hydroxyl groups. TMP was found to be slightly more stable on this type of surface than DMMP. The difference in stability is even larger if one compares this TMP system with the complex of tabun adsorbed on completely hydroxylated CaO surface model (Michalkova et al. Chem Phys Lett 438:72, 2007).

Theoretical study of the surface reactivity of alkaline earth oxides: Local density of states evaluation of the local softness

The local softness of MgO, CaO, SrO, and BaO (100) surfaces has been studied using a model based on the local density of states. In all the species, the local softness (chemical reactivity) of oxygen atoms at the surface is enhanced as compared to the bulk. The results for the local and the global softness are in agreement with the ionic pattern of the metal-oxygen bond of the series.

CO adsorption on Pd atoms deposited on MgO, CaO, SrO and BaO surfaces: density functional calculations

The adsorption properties of CO molecules adsorbed on Pd atoms supported on various sites of MgO, CaO, SrO and BaO surfaces have been studied by means of a density functional cluster model approach. Clusters were embedded in the simulated Coulomb fields that closely approximate the Madelung fields of the host surface, and ions that were the nearest neighbors to the adsorption site were allowed to relax to equilibrium. The metal Pd atoms are stabilized with different binding energies on the examined defect-free and defect-containing surfaces of the oxide supports. We considered oxide anions, neutral and charged anion vacancies located at the regular (001) surfaces. CO is used as a probe molecule to characterize where the Pd atoms are located. This is done by analyzing how the Pd–CO binding energy changes as a function of the substrate site, where the Pd atom is bound, and on the basicity of the oxide support.

CaO as a Solid Base Catalyst for Transesterification of Soybean Oil

Three different calcium oxide catalysts were synthesized from different precursors and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and temperature-programmed desorption (TPD). They were used as catalysts in the transesterification of soybean oil (SBO) for the production of fatty acid methyl esters (FAME), namely biodiesel. Calcium oxide from calcite (Cal(N)) showed the highest activity towards the transesterification of SBO. The transesterification activity of CaO was found to be highly related to the basicity of the catalysts. The catalytic activity of CaO greatly decreased when CaO was exposed to CO2. (Raman spectroscopic studies demonstrated that the formation of CaCO3 and Ca(OH)2 on the surface of CaO when CaO was exposed to room air prevented CaO from participating in the transesterification of SBO). The degree of poisoning was highly dependent on the type of precursors with Cal(N) more resistant to CO2 poisoning than CaO from aragonite (Ara(N)). Deactivated CaO catalysts could be partially regenerated. A mechanism was proposed to explain the poisoning and regenerating processes. Furthermore, whether the solid phase of CaO or dissolved CaO was the active species in the transesterification of SBO was also investigated.

Changes to Alkali Ion Content Adjacent to Crystal-Glass Interfaces

AbstractAtomic scale molecular dynamics simulations have been used to predict the location of glass modifying Na, Li and Mg species in a borosilicate Magnox type waste glass adjacent to interfaces with the (100) and (110) surfaces of MgO, CaO and SrO crystals. These simulations show a considerable increase in alkali and alkali earth concentration adjacent to specific interfaces. In particular, there are significant, systematic changes in Na, Li and Mg position and concentration as a function of both the crystal's terminating surface and composition.

Probing the role of PO stretching mode enhancement in nerve-agent sensors: Simulation of the adsorption of diisopropylfluorophosphate on the model MgO and CaO surfaces

The interactions of diisopropylfluorophosphate (DFP) with model MgO and CaO surfaces have been investigated using density functional (DFT) and Møller–Plesset second order perturbation techniques. The geometries were fully optimized at the DFT level. The calculated interaction energies and the corresponding thermodynamic properties show that DFP is physisorbed on these two model oxide surfaces and adsorption on the MgO surface is stronger. Analyses of the calculated IR and Raman spectra point to the enhancement of the PO stretching mode with respect to the isolated DFP and this property could be used to detect nerve-agents using surface-enhanced Raman spectroscopy.

Study on the generation mechanism of reactive oxygen species on calcium peroxide by chemiluminescence and UV‐visible spectra

In the present work, the generation mechanism of reactive oxygen species (ROS) on calcium peroxide (CaO(2)) was studied. A very intense chemiluminescence (CL) signal was observed when adding an aqueous solution of luminol or 2-methyl-6-(4-methoxyphenyl)-3,7-dihydroimidazo[1,2alpha]-pyrazin-3-one hydrochloride (MCLA) to a suspension of CaO(2). The ROS released on CaO(2) were thought to be oxidizing agents leading to CL, and were characterized by CL, UV-visible (UV-vis) spectra and the effective scavengers of the special ROS. From experimental results, the hydroxyl (.OH) and superoxide (.O(2) (-)) radicals were suggested to exist on the surface of CaO(2). A reaction scheme for the formation of the ROS on CaO(2) was also proposed and discussed. Of more interest was the finding that the CaO(2) which released the .OH and .O(2) (-) on the surface exhibited good transition properties compared with alkaline-earth metal peroxides of the same group (MgO(2), BaO(2)).

Partially Hydroxylated Polycrystalline Ionic Oxides: A New Route toward Electron-Rich Surfaces

Charge traps at the surface of oxide materials play a fundamental role in various chemical processes, such as the activation of supported metal clusters. In this study, combining electron paramagnetic resonance with cluster model DFT calculations, we show that excess electrons at the surface of MgO, CaO, and SrO polycrystalline materials can be generated by preparing weakly hydroxylated surfaces followed by deposition of small amounts of alkali metals. The residual OH groups present on specific sites of the partially dehydroxylated surface act as stable traps for electrons donated by the alkali metal (Na in this case) which forms a Na+ ion distant from the trapped electron. This process results in the formation of thermally stable (H+)(e-) color centers at the surface of the oxide. The procedure could be of interest for the stabilization and activation of supported metal nanoparticles with potential use in catalysis.

A density functional theory study of the adsorption of Hg and HgCl2 on a CaO(001) surface

The adsorption of mercury and mercury chloride on a CaO(001) surface was investigated by the density functional theory (DFT) by using Ca9O9 cluster embedded in an electrostatic field represented by 178 point charges at the crystal CaO lattice positions. For the mercury molecular axis normal to the surface, the mercury can only coordinate to the O2− anion and has a very weak binding energy of 19.649 kJ/mol. When the mercury chloride molecular axis is vertical to the surface, the Cl atom coordinates to the Ca2− cation and has a binding energy of 23.699 kJ/mol. When the mercury chloride molecular axis is parallel to the surface, the Hg atom coordinates to the O2− anion and has a binding energy of 87.829 kJ/mol, which means that the parallel geometry is more stable than the vertical one. The present calculations show that CaO injection could substantially reduce gaseous mercury chloride, but have no apparent effect on the mercury, which is compatible with the available experimental results. This research will provide valuable information for optimizing and selecting a sorbent for the trace element in flue gas.